Speech intelligibility in telephones with multiple microphones

ABSTRACT

The present invention is directed to improved speech intelligibility in telephones with multiple microphones. Such a telephone includes a first microphone, a second microphone, a voice activity detector (VAD), a receiver module, and a signal processor. The first microphone outputs a first audio signal, which comprises a voice component when a near-end user talks and a background noise component. The second microphone outputs a second audio signal. The VAD generates a voice activity signal responsive to a ratio between the first audio signal and the second audio signal. The voice activity signal identifies time intervals in which the voice component of the near-end user is present in the first audio signal. The receiver module receives a third audio signal, which comprises a voice component of a far-end user. The signal processor modifies the third audio signal responsive to the voice activity signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.11/653,858 to Chen, entitled “Improved Channel Decoding for WirelessTelephones with Multiple Microphones and Multiple DescriptionTransmission” and filed Jan. 17, 2007, which is a continuation-in-partof U.S. patent application Ser. No. 11/215,304 to Chen et al., entitled“Wireless Telephone with Multiple Microphones and Multiple DescriptionTransmission” and filed Aug. 31, 2005, which is a continuation-in-partof U.S. patent application Ser. No. 11/135,491 to Chen, entitled“Wireless Telephone with Adaptive Microphone Array” and filed May 24,2005, which is a continuation-in-part of U.S. patent application Ser.No. 11/065,131 to Chen, entitled “Wireless Telephone WithUni-Directional and Omni-Directional Microphones” and filed Feb. 24,2005, which is a continuation-in-part of U.S. patent application Ser.No. 11/018,921 to Chen et al., entitled “Wireless Telephone HavingMultiple Microphones” and filed Dec. 22, 2004. The entirety of each ofthese applications is hereby incorporated by reference as if fully setforth herein.

BACKGROUND

1. Field

The present invention relates generally to telecommunication devices,and in particular to telephones.

2. Background

Background noise is an inherent problem in wireless telephonecommunication. Conventional wireless telephones include a singlemicrophone that receives a near-end user's voice and outputs acorresponding audio signal for subsequent encoding and transmission tothe telephone of a far-end user. However, the audio signal output bythis microphone typically includes both a voice component and abackground noise component. As a result, the far-end user often hasdifficulty deciphering the desired voice component against the din ofthe embedded background noise component.

Conventional wireless telephones often include a noise suppressor toreduce the detrimental effects of background noise. A noise suppressorattempts to reduce the level of the background noise by processing theaudio signal output by the microphone through various algorithms. Thesealgorithms attempt to differentiate between a voice component of theaudio signal and a background noise component of the audio signal, andthen attenuate the level of the background noise component.

Conventional wireless telephones often also include a voice activitydetector (VAD) that attempts to identify and transmit only thoseportions of the audio signal that include a voice component. One benefitof VAD is that bandwidth is conserved on the telecommunication networkbecause only selected portions of the audio signal are transmitted.

In order to operate effectively, both the noise suppressor and the VADmust be able to differentiate between the voice component and thebackground noise component of the input audio signal. However, inpractice, differentiating the voice component from the background noisecomponent is difficult.

Furthermore, when a near-end user uses a telephone in a noisyenvironment, the far-end talker's voice can be difficult to understanddue to the loud acoustic background noise of the noisy environment. Aconventional technique uses the telephone microphone to pick up thebackground noise, then analyzes the noise to determine how to modify thefar-end speech signal to improve its intelligibility.

A common problem in this high background noise condition, however, isthat when the near-end user starts to talk, his voice will often betreated as noise, with the far-end user's voice being amplified orotherwise modified accordingly in an attempt to overcome this supposed“noise”. This can result in a shouting match between the near-end userand the far-end user. At the very least, the loudness of the far-enduser's voice may be modulated by the near-end user's voice, resulting inunnatural loudness fluctuation. Although a VAD can be used for thenear-end talker's voice, it is difficult to have reliable voice activitydetection when the background noise is very loud.

Another problem is that the far-end talker may have a hard timeunderstanding the near-end talker's voice since the near-end talker isin an environment with very loud background noise.

What is needed then, are telephones and methods that provide improvedspeech intelligibility. Such telephones and methods would desirablyimprove speech intelligibility, for example, when a user is located in anoisy environment.

BRIEF SUMMARY

The present invention is directed to improved speech intelligibility intelephones with multiple microphones. Embodiments of the presentinvention provide telephones and methods for improving speechintelligibility, for example, when a user is located in an environmenthaving background noise.

A method in accordance with an embodiment of the present inventionincludes a plurality of steps. In a first step, a first audio signal isoutput from a first microphone. The first audio signal comprises a voicecomponent when a near-end user talks and a background noise component.

In a second step, a second audio signal is output from a secondmicrophone.

In a third step, a ratio between the first audio signal and the secondaudio signal is monitored.

In a fourth step, a voice activity signal is generated responsive to theratio between the first audio signal and the second audio signal. Thevoice activity signal identifies time intervals in which the voicecomponent of the near-end user is present in the first audio signal.

In a fifth step, a third audio signal is received. The third audiosignal comprises a voice component of a far-end user.

In a sixth step, the third audio signal is modified responsive to thevoice activity signal.

Another embodiment of the present invention provides a telephone forimproving speech intelligibility when the telephone is used in anenvironment having background noise. The telephone includes a firstmicrophone, a second microphone, a VAD, a receiver module, and a signalprocessor.

The first microphone outputs a first audio signal. The first audiosignal comprises a voice component when a near-end user talks and abackground noise component.

The second microphone outputs a second audio signal.

The VAD monitors a ratio between the first audio signal and the secondaudio signal, and generates a voice activity signal responsive to theratio between the first audio signal and the second audio signal. Thevoice activity signal identifies time intervals in which the voicecomponent of the near-end user is present in the first audio signal.

The receiver module receives a third audio signal. The third audiosignal comprises a voice component of a far-end user.

The signal processor modifies the third audio signal responsive to thevoice activity signal.

Further embodiments and features of the present invention, as well asthe structure and operation of the various embodiments of the presentinvention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1A is a functional block diagram of the transmit path of aconventional wireless telephone.

FIG. 1B is a functional block diagram of the receive path of aconventional wireless telephone.

FIG. 2 is a schematic representation of the front portion of a wirelesstelephone in accordance with an embodiment of the present invention.

FIG. 3 is a schematic representation of the back portion of a wirelesstelephone in accordance with an embodiment of the present invention.

FIG. 4 is a functional block diagram of a transmit path of a wirelesstelephone in accordance with an embodiment of the present invention.

FIG. 5 illustrates a flowchart of a method for processing audio signalsin a wireless telephone having a first microphone and a secondmicrophone in accordance with an embodiment of the present invention.

FIG. 6 is a functional block diagram of a signal processor in accordancewith an embodiment of the present invention.

FIG. 7 illustrates a flowchart of a method for processing audio signalsin a wireless telephone having a first microphone and a secondmicrophone in accordance with an embodiment of the present invention.

FIG. 8 illustrates voice and noise components output from first andsecond microphones, in an embodiment of the present invention.

FIG. 9 is a functional block diagram of a background noise cancellationmodule in accordance with an embodiment of the present invention.

FIG. 10 is a functional block diagram of a signal processor inaccordance with an embodiment of the present invention.

FIG. 11 illustrates a flowchart of a method for processing audio signalsin a wireless telephone having a first microphone and a secondmicrophone in accordance with an embodiment of the present invention.

FIG. 12A illustrates an exemplary frequency spectrum of a voicecomponent and a background noise component of a first audio signaloutput by a first microphone, in an embodiment of the present invention.

FIG. 12B illustrates an exemplary frequency spectrum of an audio signalupon which noise suppression has been performed, in accordance with anembodiment of the present invention.

FIG. 13 is a functional block diagram of a transmit path of a wirelesstelephone in accordance with an embodiment of the present invention.

FIG. 14 is a flowchart depicting a method for processing audio signalsin a wireless telephone having a first microphone and a secondmicrophone in accordance with an embodiment of the present invention.

FIG. 15 shows exemplary plots depicting a voice component and abackground noise component output by first and second microphones of awireless telephone, in accordance with an embodiment of the presentinvention.

FIG. 16 shows an exemplary polar pattern of an omni-directionalmicrophone.

FIG. 17 shows an exemplary polar pattern of a subcardioid microphone.

FIG. 18 shows an exemplary polar pattern of a cardioid microphone.

FIG. 19 shows an exemplary polar pattern of a hypercardioid microphone.

FIG. 20 shows an exemplary polar pattern of a line microphone.

FIG. 21 shows an exemplary microphone array, in accordance with anembodiment of the present invention.

FIGS. 22A-D show exemplary polar patterns of a microphone array.

FIG. 22E shows exemplary directivity patterns of a far-field and anear-field response.

FIG. 23 shows exemplary steered and unsteered directivity patterns.

FIG. 24 is a functional block diagram of a transmit path of a wirelesstelephone in accordance with an embodiment of the present invention.

FIG. 25 illustrates a multiple description transmission system inaccordance with an embodiment of the present invention.

FIG. 26 is a functional block diagram of a transmit path of a wirelesstelephone that can be used in a multiple description transmission systemin accordance with an embodiment of the present invention.

FIG. 27 illustrates multiple versions of a voice signal transmitted by afirst wireless telephone in accordance with an embodiment of the presentinvention.

FIG. 28A, FIG. 28B, and FIG. 28C depict example trellis diagramsillustrating candidate bit sequences that may be selected by a Viterbialgorithm.

FIG. 29 is a functional block diagram of an example receive path inaccordance with an embodiment of the present invention.

FIG. 30 is a block diagram illustrating a plurality of versions of avoice signal, wherein each version includes a plurality of speechframes.

FIG. 31 is a flowchart depicting a method for improving channel decodingin accordance with an embodiment of the present invention.

FIG. 32 is a functional block diagram of a transmit path and a receivepath of a telephone in accordance with an embodiment of the presentinvention.

FIG. 33 illustrates a flowchart of a method for processing audio signalsin a telephone having a first microphone and a second microphone inaccordance with an embodiment of the present invention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers mayindicate identical or functionally similar elements. Additionally, theleft-most digit(s) of a reference number may identify the drawing inwhich the reference number first appears.

DETAILED DESCRIPTION

-   -   I. Overview Of Signal Processing Within Conventional Wireless        Telephones    -   II. Overview Of A Wireless Telephone Having Two Microphones In        Accordance With An Embodiment Of The Present Invention    -   III. Use Of Two Microphones To Perform Background Noise        Cancellation In Accordance With An Embodiment Of The Present        Invention    -   IV. Use Of Two Microphones To Perform Improved Noise Suppression        In Accordance With An Embodiment Of The Present Invention    -   V. Use Of Two Microphones To Perform Improved VAD In Accordance        With An Embodiment Of The Present Invention    -   VI. Alternative Embodiments Of The Present Invention    -   VII. Embodiments Implementing Uni-Directional Microphones    -   VIII. Microphone Arrays        -   A. Background On Microphone Arrays        -   B. Steering A Response Of A Microphone Array    -   IX. Embodiments Implementing Microphone Arrays    -   X. Multiple Description Transmission System In Accordance With        An Embodiment Of The Present Invention    -   XI. Improved Channel Decoding        -   A. Overview Of Forward Error Correction        -   B. Example Embodiments        -   C. Example Method    -   XI. Improved Speech Intelligibility        -   A. Overview        -   B. Example System For Providing Improved Speech            Intelligibility In Accordance With An Embodiment Of The            Present Invention        -   C. Example Method For Providing Improved Speech            Intelligibility In Accordance With An Embodiment Of The            Present Invention    -   XII. Conclusion

The present invention is directed to improved speech intelligibility intelephones with multiple microphones. In the detailed description thatfollows, references to “one embodiment”, “an embodiment”, “an exampleembodiment”, etc., indicate that the embodiment described may include aparticular feature, structure, or characteristic, but every embodimentmay not necessarily include the particular feature, structure, orcharacteristic. Moreover, such phrases are not necessarily referring tothe same embodiment. Further, when a particular feature, structure, orcharacteristic is described in connection with an embodiment, it issubmitted that it is within the knowledge of one skilled in the art toaffect such feature, structure, or characteristic in connection withother embodiments whether or not explicitly described.

Embodiments of the present invention provide telephones and methods forimproving speech intelligibility when a user is located in a noisyenvironment. For example, when a near-end user is located in a noisyenvironment, an embodiment of the present invention may be used toimprove the intelligibility of the speech of a far-end user as heard bythe near-end user. Another embodiment of the present invention may beused to improve the intelligibility of the speech of the near-end useras heard by the far-end user.

These embodiments, and other embodiments, are described in more detailbelow.

I. OVERVIEW OF SIGNAL PROCESSING WITHIN CONVENTIONAL WIRELESS TELEPHONES

Conventional wireless telephones use what is commonly referred to asencoder/decoder technology. The transmit path of a wireless telephoneencodes an audio signal picked up by a microphone onboard the wirelesstelephone. The encoded audio signal is then transmitted to anothertelephone. The receive path of a wireless telephone receives signalstransmitted from other wireless telephones. The received signals arethen decoded into a format that an end user can understand.

FIG. 1A is a functional block diagram of a typical transmit path 100 ofa conventional digital wireless telephone. Transmit path 100 includes amicrophone 109, an analog-to-digital (A/D) converter 101, a noisesuppressor 102, a voice activity detector (VAD) 103, a speech encoder104, a channel encoder 105, a modulator 106, a radio frequency (RF)module 107, and an antenna 108.

Microphone 109 receives a near-end user's voice and outputs acorresponding audio signal, which typically includes both a voicecomponent and a background noise component. The A/D converter 101converts the audio signal from an analog to a digital form. The audiosignal is next processed through noise suppressor 102. Noise suppressor102 uses various algorithms, known to persons skilled in the pertinentart, to suppress the level of embedded background noise that is presentin the audio signal.

Speech encoder 104 converts the output of noise suppressor 102 into achannel index. The particular format that speech encoder 104 uses toencode the signal is dependent upon the type of technology being used.For example, the signal may be encoded in formats that comply with GSM(Global Standard for Mobile Communication), CDMA (Code Division MultipleAccess), or other technologies commonly used for telecommunication.These different encoding formats are known to persons skilled in therelevant art and for the sake of brevity are not discussed in furtherdetail.

As shown in FIG. 1A, VAD 103 also receives the output of noisesuppressor 102. VAD 103 uses algorithms known to persons skilled in thepertinent art to analyze the audio signal output by noise suppressor 102and determine when the user is speaking. VAD 103 typically operates on aframe-by-frame basis to generate a signal that indicates whether or nota frame includes voice content. This signal is provided to speechencoder 104, which uses the signal to determine how best to process theframe. For example, if VAD 103 indicates that a frame does not includevoice content, speech encoder 103 may skip the encoding of the frameentirely.

Channel encoder 105 is employed to reduce bit errors that can occurafter the signal is processed through the speech encoder 104. That is,channel encoder 105 makes the signal more robust by adding redundantbits to the signal. For example, in a wireless phone implementing theoriginal GSM technology, a typical bit rate at the output of the speechencoder might be about 13 kilobits (kb) per second, whereas, a typicalbit rate at the output of the channel encoder might be about 22 kb/sec.The extra bits that are present in the signal after channel encoding donot carry information about the speech; they just make the signal morerobust, which helps reduce the bit errors.

The modulator 106 combines the digital signals from the channel encoderinto symbols, which become an analog wave form. Finally, RF module 107translates the analog wave forms into radio frequencies, and thentransmits the RF signal via antenna 108 to another telephone.

FIG. 1B is a functional block diagram of a typical receive path 120 of aconventional wireless telephone. Receive path 120 processes an incomingsignal in almost exactly the reverse fashion as compared to transmitpath 100. As shown in FIG. 1B, receive path 120 includes an antenna 128,an RF module 127, a channel decoder 125, a speech decoder 124, a digitalto analog (D/A) converter 122, and a speaker 129.

During operation, an analog input signal is received by antenna 128 andRF module 127 translates the radio frequencies into basebandfrequencies. Demodulator 126 converts the analog waveforms back into adigital signal. Channel decoder 125 decodes the digital signal back intothe channel index, which speech decoder 124 converts back into digitizedspeech. D/A converter 122 converts the digitized speech into analogspeech. Lastly, speaker 129 converts the analog speech signal into asound pressure wave so that it can be heard by an end user.

II. OVERVIEW OF A WIRELESS TELEPHONE HAVING TWO MICROPHONES INACCORDANCE WITH AN EMBODIMENT OF THE PRESENT INVENTION

A wireless telephone in accordance with an embodiment of the presentinvention includes a first microphone and a second microphone. Asmentioned above and as will be described in more detail herein, an audiosignal output by the second microphone can be used to improve thequality of an audio signal output by the first microphone or to supportimproved VAD technology.

FIGS. 2 and 3 illustrate front and back portions, respectively, of awireless telephone 200 in accordance with an embodiment of the presentinvention. As shown in FIG. 2, the front portion of wireless telephone200 includes a first microphone 201 and a speaker 203 located thereon.First microphone 201 is located so as to be close to a user's mouthduring regular use of wireless telephone 200. Speaker 203 is located soas to be close to a user's ear during regular use of wireless telephone200.

As shown in FIG. 3, second microphone 202 is located on the back portionof wireless telephone 200. Second microphone 202 is located so as to befurther away from a user's mouth during regular use than firstmicrophone 201, and preferably is located to be as far away from theuser's mouth during regular use as possible.

By mounting first microphone 201 so that it is closer to a user's mouththan second microphone 202 during regular use, the amplitude of theuser's voice as picked up by the first microphone 201 will likely begreater than the amplitude of the user's voice as picked up by secondmicrophone 202. Similarly, by so mounting first microphone 201 andsecond microphone 202, the amplitude of any background noise picked upby second microphone 202 will likely be greater than the amplitude ofthe background noise picked up by first microphone 201. The manner inwhich the signals generated by first microphone 201 and secondmicrophone 202 are utilized by wireless telephone 200 will be describedin more detail below.

FIGS. 2 and 3 show an embodiment in which first and second microphones201 and 202 are mounted on the front and back portion of a wirelesstelephone, respectively. However, the invention is not limited to thisembodiment and the first and second microphones may be located in otherlocations on a wireless telephone and still be within the scope of thepresent invention. For performance reasons, however, it is preferablethat the first and second microphone be mounted so that the firstmicrophone is closer to the mouth of a user than the second microphoneduring regular use of the wireless telephone.

FIG. 4 is a functional block diagram of a transmit path 400 of awireless telephone that is implemented with a first microphone and asecond microphone in accordance with an embodiment of the presentinvention. Transmit path 400 includes a first microphone 201 and asecond microphone 202, and a first A/D converter 410 and a second A/Dconverter 412. In addition, transmit path 400 includes a signalprocessor 420, a speech encoder 404, a channel encoder 405, a modulator406, an RF module 407, and an antenna 408. Speech encoder 404, channelencoder 405, modulator 406, RF module 407, and antenna 408 arerespectively analogous to speech encoder 104, channel encoder 105,modulator 106, RF module 107, and antenna 108 discussed with referenceto transmit path 100 of FIG. 1A and thus their operation will not bediscussed in detail below.

The method by which audio signals are processed along transmit path 400of the wireless telephone depicted in FIG. 4 will now be described withreference to the flowchart 500 of FIG. 5. The present invention,however, is not limited to the description provided by the flowchart500. Rather, it will be apparent to persons skilled in the relevantart(s) from the teachings provided herein that other functional flowsare within the scope and spirit of the present invention.

The method of flowchart 500 begins at step 510, in which firstmicrophone 201 outputs a first audio signal, which includes a voicecomponent and a background noise component. A/D converter 410 receivesthe first audio signal and converts it from an analog to digital formatbefore providing it to signal processor 420.

At step 520, second microphone 202 outputs a second audio signal, whichalso includes a voice component and a background noise component. A/Dconverter 412 receives the second audio signal and converts it from ananalog to digital format before providing it to signal processor 420.

At step 530, signal processor 420 receives and processes the first andsecond audio signals, thereby generating a third audio signal. Inparticular, signal processor 420 increases a ratio of the voicecomponent to the noise component of the first audio signal based on thecontent of the second audio signal to produce a third audio signal.

The third audio signal is then provided directly to speech encoder 404.Speech encoder 404 and channel encoder 405 operate to encode the thirdaudio signal using any of a variety of well known speech and channelencoding techniques. Modulator 406, RF module and antenna 408 thenoperate in a well-known manner to transmit the encoded audio signal toanother telephone.

As will be discussed in more detail herein, signal processor 420 maycomprise a background noise cancellation module and/or a noisesuppressor. The manner in which the background noise cancellation moduleand the noise suppressor operate are described in more detail insubsections III and IV, respectively.

III. USE OF TWO MICROPHONES TO PERFORM BACKGROUND NOISE CANCELLATION INACCORDANCE WITH AN EMBODIMENT OF THE PRESENT INVENTION

FIG. 6 depicts an embodiment in which signal processor 420 includes abackground noise cancellation module 605 and a downsampler 615(optional). Background noise cancellation module 605 receives the firstand second audio signals output by the first and second microphones 201and 202, respectively. Background noise cancellation module 605 uses thecontent of the second audio signal to cancel a background noisecomponent present in the first audio signal to produce a third audiosignal. The details of the cancellation are described below withreference to FIGS. 7 and 8. The third audio signal is sent to the restof transmit path 400 before being transmitted to the telephone of afar-end user.

FIG. 7 illustrates a flowchart 700 of a method for processing audiosignals using a wireless telephone having two microphones in accordancewith an embodiment of the present invention. Flowchart 700 is used tofacilitate the description of how background noise cancellation module605 cancels at least a portion of a background noise component includedin the first audio signal output by first microphone 201.

The method of flowchart 700 starts at step 710, in which firstmicrophone 201 outputs a first audio signal. The first audio signalincludes a voice component and a background noise component. In step720, second microphone 202 outputs a second audio signal. Similar to thefirst audio signal, the second audio signal includes a voice componentand a background noise component.

FIG. 8 shows exemplary outputs from first and second microphones 201 and202, respectively, upon which background noise cancellation module 605may operate. FIG. 8 shows an exemplary first audio signal 800 output byfirst microphone 201. First audio signal 800 consists of a voicecomponent 810 and a background noise component 820, which are alsoseparately depicted in FIG. 8 for illustrative purposes. FIG. 8 furthershows an exemplary second audio signal 850 output by second microphone202. Second audio signal 850 consists of a voice component 860 and abackground noise component 870, which are also separately depicted inFIG. 8. As can be seen from FIG. 8, the amplitude of the voice componentpicked up by first microphone 201 (i.e., voice component 810) isadvantageously greater than the amplitude of the voice component pickedup by second microphone 202 (i.e., voice component 860), and vice versafor the background noise components. As was discussed earlier, therelative amplitude of the voice component (background noise component)picked up by first microphone 201 and second microphone 202 is afunction of their respective locations on wireless telephone 200.

At step 730 (FIG. 7), background noise cancellation module 605 uses thesecond audio signal to cancel at least a portion of the background noisecomponent included in the first audio signal output by first microphone201. Finally, the third audio signal produced by background noisecancellation module 605 is transmitted to another telephone. That is,after background noise cancellation module 605 cancels out at least aportion of the background noise component of the first audio signaloutput by first microphone 201 to produce a third audio signal, thethird audio signal is then processed through the standard components orprocessing steps used in conventional encoder/decoder technology, whichwere described above with reference to FIG. 1A. The details of theseadditional signal processing steps are not described further forbrevity.

In one embodiment, background noise cancellation module 605 includes anadaptive filter and an adder. FIG. 9 depicts a background noisecancellation module 605 including an adaptive filter 901 and an adder902. Adaptive filter 901 receives the second audio signal from secondmicrophone 202 and outputs an audio signal. Adder 902 adds the firstaudio signal, received from first microphone 201, to the audio signaloutput by adaptive filter 901 to produce a third audio signal. By addingthe first audio signal to the audio signal output by adaptive filter901, the third audio signal produced by adder 902 has at least a portionof the background noise component that was present in the first audiosignal cancelled out.

In another embodiment of the present invention, signal processor 420includes a background noise cancellation module 605 and a downsampler615. In accordance with this embodiment, A/D converter 410 and A/Dconverter 412 sample the first and second audio signals output by firstand second microphones 201 and 202, respectively, at a higher samplingrate than is typically used within wireless telephones. For example, thefirst audio signal output by first microphone 201 and the second audiosignal output by second microphones 202 can be sampled at 16 kHz by A/Dconverters 410 and 412, respectively; in comparison, the typical signalsampling rate used in a transmit path of most conventional wirelesstelephones is 8 kHz. After the first and second audio signals areprocessed through background noise cancellation module 605 to cancel outthe background noise component from the first audio signal, downsampler615 downsamples the third audio signal produced by backgroundcancellation module 605 back to the proper sampling rate (e.g. 8 kHz).The higher sampling rate of this embodiment offers more precise timeslicing and more accurate time matching, if added precision and accuracyare required in the background noise cancellation module 605.

As mentioned above and as is described in more detail in the nextsubsection, additionally or alternatively, the audio signal output bythe second microphone is used to improve noise suppression of the audiosignal output by the first microphone.

IV. USE OF TWO MICROPHONES TO PERFORM IMPROVED NOISE SUPPRESSION INACCORDANCE WITH AN EMBODIMENT OF THE PRESENT INVENTION

As noted above, signal processor 420 may include a noise suppressor.FIG. 10 shows an embodiment in which signal processor 420 includes anoise suppressor 1007. In accordance with this embodiment, noisesuppressor 1007 receives the first audio signal and the second audiosignal output by first and second microphones 201 and 202, respectively.Noise suppressor 1007 suppresses at least a portion of the backgroundnoise component included in the first audio signal based on the contentof the first audio signal and the second audio signal. The details ofthis background noise suppression are described in more detail withreference to FIG. 11.

FIG. 11 illustrates a flowchart 1100 of a method for processing audiosignals using a wireless telephone having a first and a secondmicrophone in accordance with an embodiment of the present invention.This method is used to suppress at least a portion of the backgroundnoise component included in the output of the first microphone.

The method of flowchart 1100 begins at step 1110, in which firstmicrophone 201 outputs a first audio signal that includes a voicecomponent and a background noise component. In step 1120, secondmicrophone 202 outputs a second audio signal that includes a voicecomponent and a background noise component.

At step 1130, noise suppressor 1007 receives the first and second audiosignals and suppresses at least a portion of the background noisecomponent of the first audio signal based on the content of the firstand second audio signals to produce a third audio signal. The details ofthis step will now be described in more detail.

In one embodiment, noise suppressor 1007 converts the first and secondaudio signals into the frequency domain before suppressing thebackground noise component in the first audio signal. FIGS. 12A and 12Bshow exemplary frequency spectra that are used to illustrate thefunction of noise suppressor 1007.

FIG. 12A shows two components: a voice spectrum component 1210 and anoise spectrum component 1220. Voice spectrum 1210 includes pitchharmonic peaks (the equally spaced peaks) and the three formants in thespectral envelope.

FIG. 12A is an exemplary plot used for conceptual illustration purposesonly. It is to be appreciated that voice component 1210 and noisecomponent 1220 are mixed and inseparable in audio signals picked up byactual microphones. In reality, a microphone picks up a single mixedvoice and noise signal and its spectrum.

FIG. 12B shows an exemplary single mixed voice and noise spectrum beforenoise suppression (i.e., spectrum 1260) and after noise suppression(i.e., spectrum 1270). For example, spectrum 1260 is the magnitude of aFast Fourier Transform (FFT) of the first audio signal output by firstmicrophone 201.

A typical noise suppressor keeps an estimate of the background noisespectrum (e.g., spectrum 1220 in FIG. 12A), and then compares theobserved single voice and noise spectrum (e.g., spectrum 1260 in FIG.12B) with this estimated background noise spectrum to determine whethereach frequency component is predominately voice or predominantly noise.If it is considered predominantly noise, the magnitude of the FFTcoefficient at that frequency is attenuated. If it is consideredpredominantly voice, then the FFT coefficient is kept as is. This can beseen in FIG. 12B.

There are many frequency regions where spectrum 1270 is on top ofspectrum 1260. These frequency regions are considered to containpredominantly voice. On the other hand, regions where spectrum 1260 andspectrum 1270 are at different places are the frequency regions that areconsidered predominantly noise. By attenuating the frequency regionsthat are predominantly noise, noise suppressor 1007 produces a thirdaudio signal (e.g., an audio signal corresponding to frequency spectrum1270) with an increased ratio of the voice component to background noisecomponent compared to the first audio signal.

The operations described in the last two paragraphs above correspond toa conventional single-microphone noise suppression scheme. According toan embodiment of the present invention, noise suppressor 1007additionally uses the spectrum of the second audio signal picked up bythe second microphone to estimate the background noise spectrum 1220more accurately than in a single-microphone noise suppression scheme.

In a conventional single-microphone noise suppressor, background noisespectrum 1220 is estimated between “talk spurts”, i.e., during the gapsbetween active speech segments corresponding to uttered syllables. Sucha scheme works well only if the background noise is relativelystationary, i.e., when the general shape of noise spectrum 1220 does notchange much during each talk spurt. If noise spectrum 1220 changessignificantly through the duration of the talk spurt, then thesingle-microphone noise suppressor will not work well because the noisespectrum estimated during the last “gap” is not reliable. Therefore, ingeneral, and especially for non-stationary background noise, theavailability of the spectrum of the second audio signal picked up by thesecond microphone allows noise suppressor 1007 to get a more accurate,up-to-date estimate of noise spectrum 1220, and thus achieve betternoise suppression performance.

Note that the spectrum of the second audio signal should not be useddirectly as the estimate of the noise spectrum 1220. There are at leasttwo problems with using the spectrum of the second audio signaldirectly: first, the second audio signal may still have some voicecomponent in it; and second, the noise component in the second audiosignal is generally different from the noise component in the firstaudio signal.

To circumvent the first problem, the voice component can be cancelledout of the second audio signal. For example, in conjunction with a noisecancellation scheme, the noise-cancelled version of the first audiosignal, which is a cleaner version of the main voice signal, can passthrough an adaptive filter. The signal resulting from the adaptivefilter can be added to the second audio signal to cancel out a largeportion of the voice component in the second audio signal.

To circumvent the second problem, an approximation of the noisecomponent in the first audio signal can be determined, for example, byfiltering the voice-cancelled version of the second audio signal withadaptive filter 901.

The example method outlined above, which includes the use of a first andsecond audio signal, allows noise suppressor 1007 to obtain a moreaccurate and up-to-date estimate of noise spectrum 1220 during a talkspurt than a conventional noise suppression scheme that only uses oneaudio signal. An alternative embodiment of the present invention can usethe second audio signal picked up by the second microphone to helpobtain a more accurate determination of talk spurts versusinter-syllable gaps; and this will, in turn, produce a more reliableestimate of noise spectrum 1220, and thus improve the noise suppressionperformance.

For the particular example of FIG. 12B, spectrum 1260 in the noiseregions is attenuated by 10 dB resulting in spectrum 1270. It should beappreciated that an attenuation of 10 dB is shown for illustrativepurposes, and not limitation. It will be apparent to persons havingordinary skill in the art that spectrum 1260 could be attenuated by moreor less than 10 dB.

Lastly, the third audio signal is transmitted to another telephone. Theprocessing and transmission of the third audio signal is achieved inlike manner to that which was described above in reference toconventional transmit path 100 (FIG. 1A).

As mentioned above and as is described in more detail in the nextsubsection, additionally or alternatively, the audio signal output bythe second microphone is used to improve VAD technology incorporatedwithin the wireless telephone.

V. USE OF TWO MICROPHONES TO PERFORM IMPROVED VAD IN ACCORDANCE WITH ANEMBODIMENT OF THE PRESENT INVENTION

FIG. 13 is a functional block diagram of a transmit path 1300 of awireless telephone that is implemented with a first microphone and asecond microphone in accordance with an embodiment of the presentinvention. Transmit path 1300 includes a first microphone 201 and asecond microphone 202. In addition, transmit path 1300 includes an A/Dconverter 1310, an A/D converter 1312, a noise suppressor 1307(optional), a VAD 1320, a speech encoder 1304, a channel encoder 1305, amodulator 1306, an RF module 1307, and an antenna 1308. Speech encoder1304, channel encoder 1305, modulator 1306, RF module 1307, and antenna1308 are respectively analogous to speech encoder 104, channel encoder105, modulator 106, RF module 107, and antenna 108 discussed withreference to transmit path 100 of FIG. 1A and thus their operation willnot be discussed in detail below.

For illustrative purposes and not limitation, transmit path 1300 isdescribed in an embodiment in which noise suppressor 1307 is notpresent. In this example embodiment, VAD 1320 receives the first audiosignal and second audio signal output by first microphone 201 and thesecond microphone 202, respectively. VAD 1320 uses both the first audiosignal output by the first microphone 201 and the second audio signaloutput by second microphone 202 to provide detection of voice activityin the first audio signal. VAD 1320 sends an indication signal to speechencoder 1304 indicating which time intervals of the first audio signalinclude a voice component. The details of the function of VAD 1320 aredescribed with reference to FIG. 14.

FIG. 14 illustrates a flowchart 1400 of a method for processing audiosignals in a wireless telephone having a first and a second microphone,in accordance with an embodiment of the present invention. This methodis used to detect time intervals in which an audio signal output by thefirst microphone includes a voice component.

The method of flowchart 1400 begins at step 1410, in which firstmicrophone 201 outputs a first audio signal the includes a voicecomponent and a background noise component. In step 1420, secondmicrophone 202 outputs a second audio signal that includes a voicecomponent and a background noise component.

FIG. 15 shows exemplary plots of the first and second audio signalsoutput by first and second microphones 201 and 202, respectively. Plot1500 is a representation of the first audio signal output by firstmicrophone 201. The audio signal shown in plot 1500 includes a voicecomponent 1510 and a background noise component 1520. The audio signalshown in plot 1550 is a representation of the second audio signal outputby second microphone 202. Plot 1550 also includes a voice component 1560and a background noise component 1570. As discussed above, since firstmicrophone 201 is preferably closer to a user's mouth during regular usethan second microphone 202, the amplitude of voice component 1510 isgreater than the amplitude of voice component 1560. Conversely, theamplitude of background noise component 1570 is greater than theamplitude of background noise component 1520.

As shown in step 1430 of flowchart 1400, VAD 1320 detects time intervalsin which voice component 1510 is present in the first audio signal basedon the content of the first audio signal (plot 1500) and the secondaudio signal (plot 1550). By using the second audio signal in additionto the first audio signal to detect voice activity in the first audiosignal, VAD 1320 achieves improved voice activity detection as comparedto VAD technology that only monitors one audio signal. That is, theadditional information coming from the second audio signal, whichincludes mostly background noise component 1570, helps VAD 1320 betterdifferentiate what in the first audio signal constitutes the voicecomponent, thereby helping VAD 1320 achieve improved performance.

As an example, according to an embodiment of the present invention, inaddition to all the other signal features that a conventionalsingle-microphone VAD normally monitors, VAD 1320 can also monitor theenergy ratio or average magnitude ratio between the first audio signaland the second audio signal to help it better detect voice activity inthe first audio signal. This possibility is readily evident by comparingfirst audio signal 1500 and second audio signal 1550 in FIG. 15. Foraudio signals 1500 and 1550 shown in FIG. 15, the energy of first audiosignal 1500 is greater than the energy of second audio signal 1550during talk spurt (active speech). On the other hand, during the gapsbetween talk spurts (i.e. background noise only regions), the oppositeis true. Thus, the energy ratio of the first audio signal over thesecond audio signal goes from a high value during talk spurts to a lowvalue during the gaps between talk spurts. This change of energy ratioprovides a valuable clue about voice activity in the first audio signal.This valuable clue is not available if only a single microphone is usedto obtain the first audio signal. It is only available through the useof two microphones, and VAD 1320 can use this energy ratio to improveits accuracy of voice activity detection.

VI. ALTERNATIVE EMBODIMENTS OF THE PRESENT INVENTION

In an example alternative embodiment (not shown), signal processor 420includes both a background noise cancellation module and a noisesuppressor. In this embodiment, the background noise cancellation modulecancels at least a portion of a background noise component included inthe first audio signal based on the content of the second audio signalto produce a third audio signal. Then the noise suppressor receives thesecond and third audio signals and suppresses at least a portion of aresidual background noise component present in the third audio signalbased on the content of the second audio signal and the third audiosignal, in like manner to that described above. The noise suppressorthen provides a fourth audio signal to the remaining components and/orprocessing steps, as described above.

In another alternative example embodiment, a transmit path having afirst and second microphone can include a signal processor (similar tosignal processor 420) and a VAD (similar to VAD 1320). A person havingordinary skill in the art will appreciate that a signal processor canprecede a VAD in a transmit path, or vice versa. In addition, a signalprocessor and a VAD can process the outputs of the two microphonescontemporaneously. For illustrative purposes, and not limitation, anembodiment in which a signal processor precedes a VAD in a transmit pathhaving two microphones is described in more detail below.

In this illustrative embodiment, a signal processor increases a ratio ofa voice component to a background noise component of a first audiosignal based on the content of at least one of the first audio signaland a second audio signal to produce a third audio signal (similar tothe function of signal processor 420 described in detail above). Thethird audio signal is then received by a VAD. The VAD also receives asecond audio signal output by a second microphone (e.g., secondmicrophone 202). In a similar manner to that described in detail above,the VAD detects time intervals in which a voice component is present inthe third signal based on the content of the second audio signal and thethird audio signal.

In a still further embodiment, a VAD can precede a noise suppressor, ina transmit path having two microphones. In this embodiment, the VADreceives a first audio signal and a second audio signal output by afirst microphone and a second microphone, respectively, to detect timeintervals in which a voice component is present in the first audiosignal based on the content of the first and second audio signals, inlike manner to that described above. The noise suppressor receives thefirst and second audio signals and suppresses a background noisecomponent in the first audio signal based on the content of the firstaudio signal and the second audio signal, in like manner to thatdescribed above.

VII. EMBODIMENTS IMPLEMENTING UNI-DIRECTIONAL MICROPHONES

At least one of the microphones used in exemplary wireless telephone 200can be a uni-directional microphone in accordance with an embodiment ofthe present invention. As will be described in more detail below, auni-directional microphone is a microphone that is most sensitive tosound waves originating from a particular direction (e.g., sound wavescoming from directly in front of the microphone). Some of theinformation provided below concerning uni-directional andomni-directional microphones was found on the following website:<http://www.audio-technica.com/using/mphones/guide/pattern.html>.

Persons skilled in the relevant art(s) will appreciate that microphonesare often identified by their directional properties—that is, how wellthe microphones pick up sound from various directions. Omni-directionalmicrophones pick up sound from just about every direction equally. Thus,omni-directional microphones work substantially the same pointed awayfrom a subject as pointed toward it, if the distances are equal. FIG. 16illustrates a polar pattern 1600 of an omni-directional microphone. Apolar pattern is a round plot that illustrates the sensitivity of amicrophone in decibels (dB) as it rotates in front of a fixed soundsource. Polar patterns, which are also referred to in the art as “pickuppatterns” or “directional patterns,” are well-known graphical aids forillustrating the directional properties of a microphone. As shown bypolar pattern 1600 of FIG. 16, an omni-directional microphone picks upsounds equally in all directions.

In contrast to omni-directional microphones, uni-directional microphonesare specially designed to respond best to sound originating from aparticular direction while tending to reject sound that arrives fromother directions. This directional ability is typically implementedthrough the use of external openings and internal passages in themicrophone that allow sound to reach both sides of the diaphragm in acarefully controlled way. Thus, in an example uni-directionalmicrophone, sound arriving from the front of the microphone will aiddiaphragm motion, while sound arriving from the side or rear will canceldiaphragm motion.

Exemplary types of uni-directional microphones include but are notlimited to subcardioid, cardioid, hypercardioid, and line microphones.Polar patterns for example microphones of each of these types areprovided in FIG. 17 (subcardioid), FIG. 18 (cardioid), FIG. 19(hypercardioid) and FIG. 20 (line). Each of these figures shows theacceptance angle and null(s) for each microphone. The acceptance angleis the maximum angle within which a microphone may be expected to offeruniform sensitivity. Acceptance angles may vary with frequency; however,high-quality microphones have polar patterns which change very littlewhen plotted at different frequencies. A null defines the angle at whicha microphone exhibits minimum sensitivity to incoming sounds.

FIG. 17 shows an exemplary polar pattern 1700 for a subcardioidmicrophone. The acceptance angle for polar pattern 1700 spans170-degrees, measured in a counterclockwise fashion from line 1705 toline 1708. The null for polar pattern 1700 is not located at aparticular point, but spans a range of angles—i.e., from line 1718 toline 1730. Lines 1718 and 1730 are at 100-degrees from upward-pointingvertical axis 1710, as measured in a counterclockwise and clockwisefashion, respectively. Hence, the null for polar pattern 1700 spans160-degrees from line 1718 to line 1730, measured in a counterclockwisefashion.

FIG. 18 shows an exemplary polar pattern 1800 for a cardioid microphone.The acceptance angle for polar pattern 1800 spans 120-degrees, measuredin a counterclockwise fashion from line 1805 to line 1808. Polar pattern1800 has a single null 1860 located 180-degrees from upward-pointingvertical axis 1810.

FIG. 19 shows an exemplary polar pattern 1900 for a hypercardioidmicrophone. The acceptance angle for polar pattern 1900 spans100-degrees, measured in a counterclockwise fashion from line 1905 toline 1908. Polar pattern 1900 has a first null 1920 and a second null1930. First null 1920 and second null 1930 are each 110-degrees fromupward-pointing vertical axis 1910, as measured in a counterclockwiseand clockwise fashion, respectively.

FIG. 20 shows an exemplary polar pattern 2000 for a line microphone. Theacceptance angle for polar pattern 2000 spans 90-degrees, measured in acounterclockwise fashion from line 2005 to line 2008. Polar pattern 2000has a first null 2020 and a second null 2030. First null 2020 and secondnull 2030 are each 120-degrees from upward-pointing vertical axis 2010,as measured in a counterclockwise and clockwise fashion, respectively.

A uni-directional microphone's ability to reject much of the sound thatarrives from off-axis provides a greater working distance or “distancefactor” than an omni-directional microphone. Table 1, below, sets forththe acceptance angle, null, and distance factor (DF) for exemplarymicrophones of differing types. As Table 1 shows, the DF for anexemplary cardioid microphone is 1.7 while the DF for an exemplaryomni-directional microphone is 1.0. This means that if anomni-directional microphone is used in a uniformly noisy environment topick up a desired sound that is 10 feet away, a cardioid microphone usedat 17 feet away from the sound source should provide the same results interms of the ratio of desired signal to ambient noise. Among the otherexemplary microphone types listed in Table 1, the subcardioid microphoneperforms equally well at 12 feet, the hypercardioid at 20 feet, and theline at 25 feet.

TABLE 1 Properties of Exemplary Microphones of Differing Types Omni-direc- tional Subcardioid Cardioid Hypercardioid Line Acceptance — 170°120° 100°  90° Angle Null None 100° 180° 110° 120° Distance 1.0 1.2 1.72.0 2.5 Factor (DF)

VIII. MICROPHONE ARRAYS

A wireless telephone in accordance with an embodiment of the presentinvention can include at least one microphone array. As will bedescribed in more detail below, a microphone array includes a pluralityof microphones that are coupled to a digital signal processor (DSP). TheDSP can be configured to adaptively combined the audio signals output bythe microphones in the microphone array to effectively adjust thesensitivity of the microphone array to pick up sound waves originatingfrom a particular direction. Some of the information provided below onmicrophone arrays was found on the following website:<http://www.idiap.ch/˜mccowan/arrays/tutorial.pdf>.

In a similar manner to uni-directional microphones, a microphone arraycan be used to enhance the pick up of sound originating from aparticular direction, while tending to reject sound that arrives fromother directions. Like uni-directional microphones, the sensitivity of amicrophone array can be represented by a polar pattern or a directivitypattern. However, unlike uni-directional microphones, the direction inwhich a microphone array is most sensitive is not fixed. Rather, it canbe dynamically adjusted. That is, the orientation of the main lobe of apolar pattern or directivity pattern of a microphone array can bedynamically adjusted.

A. Background on Microphone Arrays

FIG. 21 is a representation of an example microphone array 2100 inaccordance with an embodiment of the present invention. Microphone array2100 includes a plurality of microphones 2101, a plurality of A/Dconverters 2103 and a digital signal processor (DSP) 2105. Microphones2101 function to convert a sound wave impinging thereon into audiooutput signals, in like manner to conventional microphones. A/Dconverters 2103 receive the analog audio output signals from microphones2101 and convert these signals to digital form in a manner well-known inthe relevant art(s). DSP 2105 receives and combines the digital signalsfrom A/D converters 2103 in a manner to be described below.

Also included in FIG. 21 are characteristic dimensions of microphonearray 2100. In an embodiment, microphones 2101 in microphone array 2100are approximately evenly spaced apart by a distance d. The distancebetween the first and last microphone in microphone array 2100 isdesignated as L. The following relationship is satisfied bycharacteristic dimensions L and d:

L=(N−1)d,  Eq. (1)

where N is the number of microphones in the array.

Characteristic dimensions d and/or L impact the response of microphonearray 2100. More particularly, the ratio of the total length ofmicrophones 2101 to the wavelength of the impinging sound (i.e., L/λ)affects the response of microphone array 2100. For example, FIGS. 22A-Dshow the polar patterns of a microphone array having different values ofL/λ, demonstrating the impact that this ratio has on the microphonearray's response.

As can be seen from FIGS. 22A-D, similar to uni-directional microphones,a microphone array has directional properties. In other words, theresponse of a microphone array to a particular sound source is dependenton the direction of arrival (DOA) of the sound waves emanating from thesound source in relation to the microphone array. The DOA of a soundwave can be understood by referring to FIG. 21. In FIG. 21, sound wavesemanating from a sound source are approximated (using the far-fieldapproximation described below) by a set of parallel wavefronts 2110 thatpropagate toward microphone array 2100 in a direction indicated by arrow2115. The DOA of parallel wavefronts 2110 can be defined as an angle φthat arrow 2115 makes with the axis along which microphones 2101 lie, asshown in the figure.

In addition to the DOA of a sound wave, the response of a microphonearray is affected by the distance a sound source is from the array.Sound waves impinging upon a microphone array can be classifiedaccording to a distance, r, these sound waves traveled in relation tothe characteristic dimension L and the wavelength of the sound B. Inparticular, if r is greater than 2 L²/λ, then the sound source isclassified as a far-field source and the curvature of the wavefronts ofthe sound waves impinging upon the microphone array can be neglected. Ifr is not greater than 2 L²/λ, then the sound source is classified as anear-field source and the curvature of the wavefronts can not beneglected.

FIG. 22E shows an exemplary directivity pattern illustrating theresponse of a microphone array for a near-field source (dotted line) anda far-field source (solid line). In the directivity pattern, the array'sresponse is plotted on the vertical axis and the angular dependence isplotted on the horizontal axis.

In a similar manner to uni-directional microphones, a maximum and aminimum sensitivity angle can be defined for a microphone array. Amaximum sensitivity angle of a microphone array is defined as an anglewithin which a sensitivity of the microphone array is above apredetermined threshold. A minimum sensitivity angle of a microphonearray is defined as an angle within which a sensitivity of themicrophone array is below a predetermined threshold.

B. Steering a Response of a Microphone Array

As mentioned above, DSP 2105 of microphone array 2100 can be configuredto combine the audio output signals received from microphones 2101 (in amanner to be described presently) to effectively steer the directivitypattern of microphone array 2100.

In general, DSP 2105 receives N audio signals and produces a singleaudio output signal, where again N is the number of microphones in themicrophone array 2100. Each of the N audio signals received by DSP 2105can be multiplied by a weight factor, having a magnitude and phase, toproduce N products of audio signals and weight factors. DSP 2105 canthen produce a single audio output signal from the collection ofreceived audio signals by summing the N products of audio signals andweight factors.

By modifying the weight factors before summing the products, DSP 2105can alter the directivity pattern of microphone array 2100. Varioustechniques, called beamforming techniques, exist for modifying theweight factors in particular ways. For example, by modifying theamplitude of the weight factors before summing, DSP 2105 can modify theshape of a directivity pattern. As another example, by modifying thephase of the weight factors before summing, DSP 2105 can control theangular location of a main lobe of a directivity pattern of microphonearray 2100. FIG. 23 illustrates an example in which the directivitypattern of a microphone array is steered by modifying the phases of theweight factors before summing. As can be seen from FIG. 23, in thisexample, the main lobe of the directivity pattern is shifted byapproximately 45 degrees.

As is well-known in the relevant art(s), beamforming techniques can benon-adaptive or adaptive. Non-adaptive beamforming techniques are notdependent on the data. In other words, non-adaptive beamformingtechniques apply the same algorithm regardless of the incoming soundwaves and resulting audio signals. In contrast, adaptive beamformingtechniques are dependent on the data. Accordingly, adaptive beamformingtechniques can be used to adaptively determine a DOA of a sound sourceand effectively steer the main lobe of a directivity pattern of amicrophone array in the DOA of the sound source. Example adaptivebeamforming techniques include, but are not limited to, Frost'salgorithm, linearly constrained minimum variance algorithms, generalizedsidelobe canceller algorithms, or the like.

It is to be appreciated that FIG. 21 is shown for illustrative purposesonly, and not limitation. For example, microphones 2101 need not beevenly spaced apart. In addition, microphone array 2100 is shown as aone-dimensional array; however two-dimensional arrays are contemplatedwithin the scope of the present invention. As a person having ordinaryskill in the art knows, two-dimensional microphone arrays can be used todetermine a DOA of a sound source with respect to two distinctdimensions. In contrast, a one-dimensional array can only detect the DOAwith respect to one dimension.

IX. EMBODIMENTS IMPLEMENTING MICROPHONE ARRAYS

In embodiments to be described below, microphone 201 and/or microphone202 of wireless telephone 200 (FIGS. 2 and 3) can be replaced with amicrophone array, similar to microphone array 2100 shown in FIG. 21.

FIG. 24 is an example transmit path 2400 of a wireless telephoneimplemented with a first microphone array 201′ and a second microphonearray 202′. First microphone array 201′ and second microphone array 202′function in like manner to exemplary microphone array 2100 (FIG. 21)described above. In particular, microphones 2401 a-n and 2411 a-nfunction to convert sound waves impinging thereon into audio signals.A/D converters 2402 a-n and 2412 a-n function to convert the analogaudio signals received from microphones 2401 a-n and 2411 a-n,respectively, into digital audio signals. DSP 2405 receives the digitalaudio signals from A/D converters 2402 a-n and combines them to producea first audio output signal that is sent to signal processor 420′.Similarly, DSP 2415 receives the digital audio signals from A/Dconverters 2412 a-n and combines them to produce a second audio outputsignal that is sent to signal processor 420′.

The remaining components in transmit path 2400 (namely, signal processor420′, speech encoder 404′, channel encoder 405′, modulator 406′, RFmodule 407′ and antenna 408′) function in substantially the same manneras the corresponding components discussed with reference to FIG. 4.Accordingly, the functionality of the remaining components is notdiscussed further.

In an embodiment of the present invention, DSP 2405, using adaptivebeamforming techniques, determines a DOA of a voice of a user of awireless telephone based on the digital audio signals received from A/Dconverters 2402 a-n. DSP 2405 then adaptively combines the digital audiosignals to effectively steer a maximum sensitivity angle of microphonearray 201′ so that the mouth of the user is within the maximumsensitivity angle. In this way, the single audio signal output by DSP2405 will tend to include a cleaner version of the user's voice, ascompared to an audio signal output from a single microphone (e.g.,microphone 201). The audio signal output by DSP 2405 is then received bysignal processor 420′ and processed in like manner to the audio signaloutput by microphone 201 (FIG. 4), which is described in detail above.

In another embodiment of the present invention, DSP 2415 receives thedigital audio signals from A/D converters 2412 a-n and, using adaptivebeamforming techniques, determines a DOA of a voice of a user of thewireless telephone based on the digital audio signals. DSP 2415 thenadaptively combines the digital audio signals to effectively steer aminimum sensitivity angle of microphone array 202′ so that the mouth ofthe user is within the minimum sensitivity angle. In this way, thesingle audio signal output by DSP 2415 will tend to not include theuser's voice; hence the output of DSP 2415 will tend to include a purerversion of background noise, as compared to an audio signal output froma single microphone (e.g., microphone 202). The audio signal output byDSP 2415 is then received by signal processor 420′ and processed in likemanner to the audio signal output by microphone 202 (FIG. 4), which isdescribed in detail above.

In most situations background noise is non-directional—it issubstantially the same in all directions. However, in some situations asingle noise source (e.g., a jackhammer or ambulance) accounts for amajority of the background noise. In these situations, the backgroundnoise is highly directional. In an embodiment of the invention, DSP 2405is configured to determine a DOA of a highly directional backgroundnoise source. DSP 2405 is further configured to adaptively combine thedigital audio signals to effectively steer a minimum sensitivity angleof microphone array 201′ so that the highly directional background noisesource is within the minimum sensitivity angle. In this way, microphonearray 201′ will tend to reject sound originating from the DOA of thehighly directional background noise source. Hence, microphone array 201′will consequently pick up a purer version of a user's voice, as comparedto a single microphone (e.g., microphone 201).

In another embodiment, DSP 2415 is configured to determine a DOA of ahighly directional background noise source. DSP 2415 is furtherconfigured to adaptively combine the digital audio signals from A/Dconverters 2412 a-n to effectively steer a maximum sensitivity angle ofmicrophone array 202′ so that the highly directional background noisesource is within the maximum sensitivity angle. In this way, microphonearray 202′ will tend to pick-up sound originating from the DOA of thehighly directional background noise source. Hence, microphone array 202′will consequently pick up a purer version of the highly directionalbackground noise, as compared to a single microphone (e.g., microphone202).

In a further embodiment (not shown), a wireless telephone includes afirst and second microphone array and a VAD. In this embodiment, a DSPis configured to determine a DOA of a highly directional backgroundnoise and a DOA of a user's voice. In addition, in a similar fashion tothat described above, the VAD detects time intervals in which a voicecomponent is present in the audio signal output by the first microphonearray. During time intervals in which a voice signal is present in theaudio signal output from the first microphone array, a DSP associatedwith the second microphone array adaptively steers a minimum sensitivityangle of the second microphone array so that the mouth of the user iswithin the minimum sensitivity angle. During time intervals in which avoice signal is not present in the audio signal output from the firstmicrophone array, a DSP associated with the second microphone arrayadaptively steers a maximum sensitivity angle of the second microphonearray so that the highly directional background noise source is withinthe maximum sensitivity angle. In other words, the second microphonearray, with the help of the VAD, adaptively switches between thefollowing: (i) rejecting the user's voice during time intervals in whichthe user is talking; and (ii) preferentially picking up a highlydirectional background noise sound during time intervals in which theuser is not talking. In this way, the second microphone array can pickup a purer version of background noise as compared to a singlemicrophone.

It is to be appreciated that the embodiments described above are meantfor illustrative purposes only, and not limitation. In particular, it isto be appreciated that the term “digital signal processor,” “signalprocessor” or “DSP” used above and below can mean a single DSP, multipleDSPs, a single DSP algorithm, multiple DSP algorithms, or combinationsthereof. For example, DSP 2405, DSP 2415 and/or signal processor 420′(FIG. 24) can represent different DSP algorithms that function within asingle DSP. Additionally or alternatively, various combinations of DSP2405, DSP 2415 and/or signal processor 420′ can be implemented in asingle DSP or multiple DSPs as is known by a person skilled in therelevant art(s).

X. MULTIPLE DESCRIPTION TRANSMISSION SYSTEM IN ACCORDANCE WITH ANEMBODIMENT OF THE PRESENT INVENTION

FIG. 25 illustrates a multiple description transmission system 2500 thatprovides redundancy to combat transmission channel impairments inaccordance with embodiments of the present invention. Multipledescription transmission system 2500 includes a first wireless telephone2510 and a second wireless telephone 2520. First wireless telephone 2510transmits multiple versions 2550 of a voice signal to second wirelesstelephone 2520.

FIG. 26 is a functional block diagram illustrating an example transmitpath 2600 of first wireless telephone 2510 and an example receive path2650 of second wireless telephone 2520. As shown in FIG. 26, firstwireless telephone 2510 comprises an array of microphones 2610, anencoder 2620, and a transmitter 2630. Each microphone in microphonearray 2610 is configured to receive voice input from a user (in the formof a sound pressure wave) and to produce a voice signal correspondingthereto. Microphone array 2610 can be, for example, substantially thesame as microphone array 2100 (FIG. 21). Encoder 2620 is coupled tomicrophone array 2610 and configured to encode each of the voicesignals. Encoder 2620 can include, for example, a speech encoder andchannel encoder similar to speech encoder 404 and channel encoder 405,respectively, which are each described above with reference to FIG. 4.Additionally, encoder 2620 may optionally include a DSP, similar to DSP420 (FIG. 4).

Transmitter 2630 is coupled to encoder 2620 and configured to transmiteach of the encoded voice signals. For example, FIG. 25 conceptuallyillustrates an example multiple description transmission system. In FIG.25, first wireless telephone 2510 transmits a first signal 2550A and asecond signal 2550B to second wireless telephone 2520. It is to beappreciated, however, that first wireless telephone 2510 can transmitmore than two signals (e.g., three, four, five, etc.) to second wirelesstelephone 2520. Transmitter 2630 of first wireless telephone 2510 caninclude, for example, a modulator, an RF module, and an antenna similarto modulator 406, RF module 407, and antenna 408, respectively, which,as described above with reference to FIG. 4, collectively function totransmit encoded voice signals.

In alternative embodiments, first wireless telephone 2510 can includemultiple encoders and transmitters. For instance, first wirelesstelephone 2510 can include multiple transmit paths similar to transmitpath 100 (FIG. 1A), where each transmit path corresponds to a singlemicrophone of microphone array 2610 of first wireless telephone 2510.

As shown in receive path 2650 of FIG. 26, second wireless telephone 2520comprises a receiver module 2660, a decoder 2670, and a speaker 2680.Receiver module 2660 is configured to receive transmitted signals 2550(FIG. 25). For example, receiver module 2660 can include an antenna, anRF module, and a demodulator similar to antenna 128, RF module 127, anddemodulator 126, respectively, which, as described above with referenceto FIG. 1B, collectively function to receive transmitted signals.Decoder 2670 is coupled to receiver module 2660 and configured to decodethe signals received by receiver module 2660, thereby producing anoutput signal. For example, decoder 2670 can include a channel decoderand speech decoder similar to channel decoder 125 and speech decoder124, respectively, which, as described above with reference to FIG. 1B,collectively function to decode a received signal. Additionally, decoder2670 may optionally include a DSP.

Speaker 2680 receives the output signal from decoder 2670 and produces apressure sound wave corresponding thereto. For example, speaker 2680 canbe similar to speaker 129 (FIG. 1B). Additionally, a power amplifier(not shown) can be included before speaker 2680 (or speaker 129) toamplify the output signal before it is sent to speaker 2680 (speaker129) as would be apparent to a person skilled in the relevant art(s).

In a first embodiment of the present invention, decoder 2670 is furtherconfigured to perform two functions: (i) time-align the signals receivedby receiver module 2660, and (ii) combine the time-aligned signals toproduce the output signal. As is apparent from FIG. 21, due to thespatial separation of the microphones in a microphone array, a soundwave emanating from the mouth of a user may impinge upon each microphonein the array at different times. For example, with reference to FIG. 21,parallel wave fronts 2110 will impinge upon the left-most microphone ofmicrophone array 2100 before it impinges upon the microphone separatedby a distance d from the left-most microphone. Since there can be atime-delay with respect to when the sound waves impinge upon therespective microphones in microphone array 2610, there will be acorresponding time-delay with respect to the audio signals output by therespective microphones. Decoder 2670 of second wireless telephone 2520can compensate for this time-delay by time-aligning the audio signals.

For example, FIG. 27 shows a first audio signal S1 and a second audiosignal S2 corresponding to the output of a first and second microphone,respectively, of first wireless telephone 2510. Due to the relativelocation of the microphones on first wireless telephone 2510, secondaudio signal S2 is time-delayed by an amount t1 compared to first audiosignal S1. Decoder 2670 of second wireless telephone 2520 can beconfigured to time-align first audio signal S1 and second audio signalS2, for example, by time-delaying first audio signal S1 by an amountequal to t1.

As mentioned above, according to the first embodiment, decoder 2670 ofsecond wireless telephone 2520 is further configured to combine thetime-aligned audio signals. Since the respective voice components offirst audio signal S1 and second audio signal S2 are presumably nearlyidentical but the respective noise components in each audio signal arenot, the voice components will tend to add-up in phase, whereas thenoise components, in general, will not. In this way, by combining theaudio signals after time-alignment, the combined output signal will havea higher signal-to-noise ratio than either first audio signal S1 orsecond audio signal S2.

In a second embodiment of the present invention, decoder 2670 of secondwireless telephone 2520 is configured to perform the followingfunctions. First, decoder 2670 is configured to detect a direction ofarrival (DOA) of a sound wave emanating from the mouth of a user offirst wireless telephone 2510 based on transmitted signals 2550 receivedby receiver module 2660 of second wireless telephone 2520. Decoder 2670can determine the DOA of the sound wave in a similar manner to thatdescribed above with reference to FIGS. 21 through 24.

Second, decoder 2670, which as mentioned above may optionally include aDSP, is configured to adaptively combine the received signals based onthe DOA to produce the output signal. By adaptively combining thereceived signals based on the DOA, decoder 2670 of second wirelesstelephone 2520 can effectively steer a maximum sensitivity angle ofmicrophone array 2610 of first wireless telephone 2510 so that the mouthof the user of first wireless telephone 2510 is within the maximumsensitivity angle. As defined above, the maximum sensitivity angle is anangle within which a sensitivity of microphone array 2610 is above athreshold.

In a third embodiment of the present invention, for each voice frame ofthe signals received by receiver module 2660, decoder 2670 of secondwireless telephone 2520 is configured to perform the followingfunctions. First, decoder 2670 is configured to estimate channelimpairments (e.g., bit errors and frame loss). That is, decoder 2670 isconfigured to determine the degree of channel impairments for each voiceframe of the received signals. For example, for a given frame, decoder2670 can estimate whether the channel impairments exceed a threshold.The estimate can be based on signal-to-noise ratio (S/N) orcarrier-to-interference ratio (C/I) of a channel, the bit error rate,block error rate, frame error rate, and or the like. Second, decoder2670 is configured to decode a received signal with the least channelimpairments, thereby producing the output signal for the respectivevoice frames.

By adaptively decoding the signal with the least channel impairments forthe respective voice frames, decoder 2670 is configured to decode thebest signal for a given time. That is, at different times the multipleversions 2550 of the voice signal transmitted by first wirelesstelephone 2510 may be subject to different channel impairments. Forexample, for a given voice frame, first signal 2550A may have lesschannel impairments than second signal 2550B. During this voice frame,decoding first signal 2550A may lead to a cleaner and better qualityvoice signal. However, during a subsequent voice frame, first signal2550A may have more channel impairments than second signal 2550B. Duringthis subsequent voice frame, decoding second signal 2550B may lead to acleaner and better quality voice signal.

In a fourth embodiment of the present invention, for each voice frame ofthe signals received by receiver module 2660, decoder 2670 is configuredto estimate channel impairments and dynamically discard those receivedsignals having a channel impairment worse than a threshold. Then,decoder 2670 is further configured to combine the non-discarded receivedsignals according to either the first or second embodiment describedabove. That is, decoder 2670 can be configured to time-align and combinethe non-discarded received signals in accordance with the firstembodiment. Alternatively, decoder 2670 can be configured to combine thenon-discarded received signals to effectively steer microphone array2610 of first wireless telephone 2510 in accordance with the secondembodiment.

In a fifth embodiment of the present invention, encoder 2620 of firstwireless telephone 2510 is configured to encode the voice signals atdifferent bit rates. For example, encoder 2620 can be configured toencode one of the voice signals at a first bit rate (“a main channel”)and each of the other voice signals at a bit rate different from thefirst bit rate (“auxiliary channels”). The main channel can be encodedand transmitted, for example, at the same bit rate as a conventionalsingle-channel wireless telephone (e.g., 22 kilobits per second);whereas the auxiliary channels can be encoded and transmitted, forexample, at a bit rate lower than a conventional single-channel wirelesstelephone (e.g., 8 kilobits per second or 4 kilobits per second). Inaddition, different ones of the auxiliary channels can be encoded andtransmitted at different bit rates. For example, a first of theauxiliary channels can be encoded and transmitted at 8 kilobits persecond; whereas a second and third auxiliary channel can be encoded andtransmitted at 4 kilobits per second. Decoder 2670 of second wirelesstelephone 2520 then decodes the main and auxiliary channels according toone of the following two examples.

In a first example, for each voice frame of the transmitted signals,decoder 2670 of second wireless telephone 2520 is configured to estimatechannel impairments. A channel is corrupted if the estimated channelimpairments for that channel exceed a threshold. If (i) the main channelis corrupted by channel impairments, and if (ii) at least one of theauxiliary channels is not corrupted by channel impairments, then thedecoder is configured to decode one of the auxiliary channels to producethe output signal.

In a second example, decoder 2670 uses the main channel and one of theauxiliary channels to improve the performance of a frame erasureconcealment algorithm. Frame erasure occurs if the degree of channelimpairments in a given voice frame exceeds a predetermined threshold.Rather than output no signal during a voice frame that has been erased,which would result in no sound during that voice frame, some decodersemploy a frame erasure concealment algorithm to conceal the occurrenceof an erased frame.

A frame erasure concealment algorithm attempts to fill the gap in soundby extrapolating a waveform for the erased frame based on the waveformthat occurred before the erased frame. Some frame erasure concealmentalgorithms use the side information (e.g., predictor coefficients, pitchperiod, gain, etc.) to guide the waveform extrapolation in order tosuccessfully conceal erased frames. An example frame erasure concealmentalgorithm is disclosed in U.S. patent application Ser. No. 10/968,300 toThyssen et al., entitled “Method for Packet Loss And/Or Frame ErasureConcealment In A Voice Communication System,” filed Oct. 20, 2004, theentirety of which is incorporated by reference herein.

In this second example, for each voice frame of the transmitted signals,decoder 2670 is configured to estimate channel impairments. If (i) theside information of the main channel is corrupted, and if (ii) thecorresponding side information of at least some of the auxiliarychannels channel is not corrupted, then decoder 2670 is configured touse both the main channel and one of the auxiliary channels to improveperformance of a frame erasure concealment algorithm in the productionof the output signal. By using uncorrupted side information from one ofthe auxiliary channels, the frame erasure concealment algorithm can moreeffectively conceal an erased frame.

XI. IMPROVED CHANNEL DECODING

As described above, a multiple-description transmission system can beused to combat transmission channel impairments. In addition to theseveral advantages and embodiments mentioned above, themultiple-description transmission system can also provide improvedchannel decoding. However, before describing embodiments that canimprove channel decoding, a brief overview of forward error correction(FEC) techniques is given.

A. Overview of Forward Error Correction

A wireless voice signal can be corrupted during transmission betweenwireless telephones. Often FEC techniques are employed to correct errorsthat occur due to the corruption of transmitted signals. To implement anFEC technique, operations must be performed on both the encoding anddecoding sides of the wireless communications process. On the encodingside, an FEC technique adds redundant information to data that is to betransmitted over a channel. By using this redundant information,transmission errors can be corrected. The process of adding theredundant information to the data is called channel encoding. Forexample, as mentioned above with reference to FIG. 1A, channel encoder105 of transmit path 100 can add redundant information to digitized bitsthat are to be transmitted to another telephone. As is well-known in theart, convolutional coding is a common way to add redundant informationto the data being transmitted to achieve FEC. A convolutional encodermakes the adjacent transmitted data symbols inter-dependent.

On the decoding side, one method for decoding convolutionally encodeddata is maximum-likelihood sequence estimation (MLSE) that performs softdecisions while searching for a sequence that minimizes a distancemetric in a trellis that characterizes the memory or inter-dependence ofthe transmitted data symbols. As is well-known in the art, the Viterbialgorithm is typically used in channel decoding to reduce the number ofpossible sequences in the trellis search when new symbols are received.For example, a Viterbi algorithm could be implemented within channeldecoder 125 of FIG. 1B.

During the channel decoding process, a typical Viterbi algorithmreceives the digitized bits of each speech frame. If no errors occurred,the digitized bits received by the Viterbi algorithm for a particularspeech frame would exactly represent the state of the encoder inencoding that speech frame. However, since errors are likely to occur,the digitized bits received by the Viterbi algorithm may not berepresentative of the message encoded by the encoder. Accordingly, theViterbi algorithm attempts to select a sequence of bits that most likelyrepresent the state of the encoder in encoding the message. In this way,if the Viterbi algorithm is successful in selecting a bit sequence thatis representative of that used to encode the message, the errors thatoccurred during the transmission of the message would be corrected. TheViterbi algorithm begins this error correction process by developing alist of candidate bit sequences that potentially represent the intendedmessage.

FIG. 28A depicts a first candidate path 2801 (bit sequence) through atrellis, FIG. 28B depicts a second candidate path 2803 (bit sequence)through the trellis, and FIG. 28C depicts a third candidate path 2805(bit sequence) through the trellis. Each candidate path may have adistance measure (or cost function). The conventional Viterbi algorithmselects the path with the lowest distance measure (or cost function).

As mentioned above, the Viterbi algorithm selects the optimal bitsequence based on a minimization of the distance between successivestates of a given speech frame—i.e., the optimal bit sequence isselected based on characteristics of the digitized bits. In other words,in a typical Viterbi algorithm, the selection of the most likely messageencoded by the encoder has nothing to do with the characteristics of thespeech that the message represents. In contrast to a typical Viterbialgorithm, an embodiment of the present invention can use redundancy inthe multiple-description transmission of a speech signal to improvechannel decoding.

B. Example Embodiments

As mentioned above, a multiple-description transmission system inaccordance with an embodiment of the present invention transmitsmultiple versions of the channel encoded digitized bits. For example,FIG. 25 illustrates multiple signals 2550A-B being transmitted betweenfirst wireless telephone 2510 and second wireless telephone 2520. Anembodiment of the present invention can use redundancy in the multipleversions to improve channel decoding. In another embodiment, redundancyin certain parameters of speech can also be used to improve channeldecoding.

In U.S. Patent Application Publication Number 2006/0050813, “Method andSystem for Decoding Video, Voice, and Speech Data Using Redundancy,” byA. Heiman and M.-S. Arkady, a method is described where the physicalconstraints of a speech signal, such as the continuity of certain speechparameters (e.g. gain, pitch period, and line spectrum pairs (LSPs),etc.) from frame to frame, are used to help identify an optimal bitsequence from many candidate sequences in a typical Viterbi algorithmtrellis search. The entirety of U.S. Patent Application PublicationNumber 2006/0050813 is incorporated by reference herein. In the exampleembodiments of the present invention, the inherent redundancy in suchspeech parameters due to multiple description transmission of the speechsignal is used either alone or together with the physical constraintsfrom frame to frame to help identify an optimal bit sequence from manycandidate sequences in a Viterbi search.

FIG. 29 is a functional block diagram of a receive path 2900 that can beused in a first embodiment of the present invention. Receive path 2900includes a receiver module 2902, a channel decoder 2904, a speechdecoder 2906, and a speaker 2908.

Receiver module 2902 receives a plurality of versions of a voice signal.For example, as shown in FIG. 30, receiver module 2902 can receive afirst voice signal 3010A, a second voice signal 3010B, and a third voicesignal 3010C. Each version of voice signals 3010 includes a plurality ofspeech frames labeled speech frame 1 through speech frame N. For eachversion of voice signal 3010, commonly labeled speech frames representtime aligned speech frames. For example, speech frame 2 for voice signal3010A, speech frame 2 for voice signal 3010B, and speech frame 2 forvoice signal 3010C are samplings of sounds that occurred oversubstantially identical durations of time. Speech frames that occur oversubstantially identical durations of time are referred to herein ascorresponding speech frames. Thus, for example, speech frame 2 for voicesignal 3010A is a corresponding speech frame to speech frame 2 for voicesignal 3010B.

Channel decoder 2904 is configured to decode a speech parameterassociated with a speech frame of one of the plurality of versions ofthe voice signal. For example, channel decoder 2904 can decode a speechparameter in speech frame 2 from first voice signal 3010A. As describedabove, decoding the speech parameter includes selecting an optimal bitsequence from a plurality of candidate bit sequences. That is, channeldecoder 2904 can implement a Viterbi algorithm in the channel decodingprocess. However, in this embodiment the selection of the optimal bitsequence is also based in part on a corresponding speech frame fromanother version of the voice signal. For example, in addition to speechframe 2 from first voice signal 3010A, channel decoder 2904 can useinformation from speech frame 2 from second voice signal 3010B and/orspeech frame 2 from third voice signal 3010C in the selection of theoptimal bit sequence.

By using information from the corresponding speech frame from anotherversion of the voice signal, channel decoder 2904 can use redundancyinherent in the multiple-description transmission to improve theselection of the optimal bit sequence. That is, each of the multipleversions of the voice signal transmitted between the first and secondtelephone will be affected differently by channel impairments. However,the underlying parameters (e.g. pitch period, gain, and LSPs) of themultiple versions of the transmitted speech signal should besubstantially similar for speech frames that cover substantiallyidentical time period. Therefore, if a decoding system such as the onedescribed in the aforementioned U.S. Patent Application PublicationNumber 2006/0050813 is used to decode each of the multiple versions ofthe speech signal, then, when decoding one of the speech parameters inone of the received speech signal versions, the same speech parametersin a corresponding speech frame in other received speech signal versionscan be used to help select the correct speech parameter.

For example, some of the bits corresponding to the pitch periodparameter in speech frame 2 of the first voice signal 3010A may becorrupted by channel impairments; whereas, the corresponding bits inspeech frame 2 of the second voice signal 3010B and/or third voicesignal 3010C may not be corrupted. However, since signals 3010A, 3010B,and 3010C are just multiple description versions of the same underlyingspeech signal spoken at the transmitter side, given a particular voicedspeech frame, these three versions of the speech signal should havepitch period parameters that are either identical or very close to eachother. Therefore, there is tremendous redundancy between the same speechparameters in corresponding speech frames of the multiple receivedversions of the speech signal.

By exploiting such speech parameter redundancy across multiple receivedversions of the speech signal in addition to exploiting the “physicalconstraints” of the same speech parameter in successive frames in time(as is done in the system described in the aforementioned U.S. PatentApplication Publication Number 2006/0050813), one can achieve even morereliable channel decoding of the speech signal than is possible byexploiting the “physical constraints” alone. In the example above, byusing the pitch period information from speech frame 2 of the receivedspeech signals 3010A, 3010B, and 3010C, channel decoder 2904 can morereliably select an optimal bit sequence that is representative of theencoded message. The same idea can be applied to other speech parameterssuch as the gain and the LSPs.

Referring again to FIG. 29, after channel decoder 2904 selects anoptimal bit sequence for the speech parameter, speech decoder 2906decodes at least one of the plurality of versions of the voice signalbased on the speech parameter to generate an output signal. Speaker 2908receives the output signal and produces a sound pressure wavecorresponding thereto.

In the first embodiment, channel decoder 2904 selects the optimal bitsequence based in part on the corresponding speech frame from anotherversion of the voice signal. In a second embodiment of the presentinvention, channel decoder 2904 selects the optimal bit sequence based(i) in part on the corresponding speech frame from the other version ofthe voice signal and (ii) in part on a previous speech frame from atleast one of the plurality of versions of the voice signal. For example,in the first embodiment the selection of the optimal bit sequence forspeech frame 2 of first voice signal 3010A can be based on speech frame2 from second signal 3010B and/or third signal 3010C. In the secondembodiment, this selection can also be based on, for example,information in speech frame 1 from voice signals 3010A, 3010B, and/or3010C. In this way, “physical constraints” of the speech parameters canbe used in addition to the redundancies in the speech parameters toimprove the selection of the optimal bit sequence.

That is, some speech parameters—including, but not limited to, pitchperiod, gain, and spectral envelop shape—have an inherent redundancy dueto the manner in which the speech parameters are generated duringnatural speech. For example, pitch period is a speech parameter thatvaries relatively slowly over time—i.e., it does not change abruptlyduring voiced segments of speech. Such a physical constraint is a formof redundancy.

By examining the value of these speech parameters in previous speechframes, channel decoder 2904 can use this redundancy to make a betterselection of the optimal bit sequence. For instance, if the value of thepitch period in speech frame 1 for each of voice signals 3010 is verydifferent from the value of the pitch period in speech frame 2 of firstvoice signal 3010A and frame 2 is in a voiced segment of speech, it isan indication that the information in speech frame 2 of first voicesignal 3010A is probably corrupted. Based on this indication, channeldecoder 2904 can use more reliable information (i.e., uncorruptedinformation) from speech frame 2 of second voice signal 3010B and/orthird voice signal 3010C in its selection of the optimal bit sequence.

C. Example Method

FIG. 31 is a flowchart 3100 illustrating a method for improving channeldecoding in a multiple-description transmission system in accordancewith an embodiment of the present invention. Flowchart 3100 begins at astep 3110 in which a plurality of versions of a voice signal arereceived, wherein each version includes a plurality of speech frames.For example, receiver module 2902 can receive the plurality of versionsof the voice signal, which can be similar to voice signals 3010.

In a step 3120, a speech parameter associated with a speech frame of oneof the plurality of versions of the voice signal is decoded. Decodingthe speech parameter associated with the speech frame includes selectingan optimal bit sequence from a list of candidate bit sequences, whereinthe selection of the optimal bit sequence is based in part on acorresponding speech frame from another version of the plurality ofversions of the voice signal. In addition, selection of the optimal bitsequence can also be based on a previous speech frame from at least oneof the plurality of versions of the voice signal.

In a step 3130, at least one of the plurality of versions of the voicesignal is decoded based on the speech parameter to produce an outputsignal. For example, referring to FIG. 30, speech decoder 2906 candecode at least one of first voice signal 3010A, second voice signal3010B, and/or third voice signal 3010C to produce the output signal.

In a step 3140, a sound pressure wave corresponding to the decodedoutput signal is produced. For example, the sound pressure wave can beproduced by speaker 2908. Additionally, as would be understood by aperson skilled in the relevant art(s), a power amplifier can be used toamplify the decoded output signal before it is converted into a soundpressure wave by the speaker.

XI. IMPROVED SPEECH INTELLIGIBILITY

Embodiments of the present invention relate to implementing a telephone(such as a mobile telephone, cordless telephone, desk telephone, or thelike) with at least two microphones. The two or more microphones may beused to improve speech intelligibility—e.g., when a user uses thetelephone in a noisy environment.

A. Overview

When a near-end user uses a telephone in a noisy environment, a far-enduser's voice can be difficult to understand due to the loud acousticbackground noise of the noisy environment. As used herein, a “near-enduser” refers to a user of a first telephone, and a “far-end user” refersto a user of a second telephone who communicates with the near-end uservia a telecommunications coupling between the first telephone and thesecond telephone.

A conventional technique attempts to improve the intelligibility of thefar-end user's speech as heard by the near-end user when the near-enduser is in a noisy environment. This conventional technique is describedin U.S. Patent Publication 2004/0057586 A1 to Licht, the entirety ofwhich is incorporated by reference herein. According to thisconventional technique, the microphone of the near-end user's telephonepicks up the background noise, and the near-end user's telephonereceives a speech signal from the far-end user's telephone correspondingto the far-end user's speech. In accordance with this conventionaltechnique, the background noise is then analyzed with respect to thereceived speech signal to determine how to modify the received speechsignal to improve its intelligibility. There are at least two exampleways in which the received speech signal may be modified.

In one conventional example, a signal processor included on the near-enduser's telephone may boost the amplitude of the entire received speechsignal. In this way, the received speech signal is converted into asound wave having a boosted amplitude.

In another conventional example, the signal processor of the near-endtelephone may boost the amplitude of specific frequency ranges of thereceived speech. In some situations, the high frequencies in the speechof the far-end user may be overpowered by the background noise in thenear-end user's environment. In such situations, a Fast FourierTransform (FFT) may be performed on the far-end user's speech signal(received from the far-end user's telephone) and the background noisesignal (picked up by the microphone of the near-end user's telephone).The FFT converts the far-end user's speech signal and the backgroundnoise signal from the time domain to the frequency domain. The signalprocessor on the near-end user's telephone may then boost the far-enduser's speech signal in frequency ranges where the far-end user's speechis overpowered by the background noise.

Thus, in accordance with either of the foregoing convention examples,the near-end user may be more likely to understand the far-end user'sspeech despite the noisy environment in which the near-end user islocated. Although this conventional technique provides improvements,there are several problems.

A first problem is that the near-end user's speech may be treated asnoise. As a result, the far-end user's voice will be boosted tocompensate for the additional “noise” (i.e., the near-end user'sspeech). This may result in a shouting match—as the conventionalalgorithm may boost the far-end user's speech to overcome the near-enduser's speech, while the near-end user may attempt to talk louder toovercome the far-end user's boosted speech if the near-end user tries tointerrupt the far-end user. Although the near-end user's telephone maybe equipped with a VAD to detect when the near-end user talks, it isdifficult to have reliable voice activity detection when the backgroundnoise is very loud.

A second problem is that, even if the near-end user can understand thefar-end user's speech due to the use of the conventional method toimprove the intelligibility of the far-end user's speech, the far-enduser may have a hard time understanding the near-end user's voicebecause the near-end user is in a noisy environment (e.g., thesignal-to-noise ratio is low). As a result, it still may not be possibleto have a successful two-way conversation.

Embodiments of the present invention improve the situation by using twoor more microphones in the near-end user's telephone. Consequently, thenear-end user's telephone can, for example, provide improved voiceactivity detection (to overcome the first problem), and increase thesignal-to-noise ratio of the near-end user's speech signal (to overcomethe second problem).

B. Example System for Providing Improved Speech Intelligibility inAccordance with an Embodiment of the Present Invention

FIG. 32 depicts a functional block diagram illustrating an exampletransmit path 3200 and an example receive path 3240 for providingimproved speech intelligibility in accordance with an embodiment of thepresent invention. Transmit path 3200 and receive path 3240 are includedin a near-end user's telephone (such as telephone 200 of FIGS. 2-3).

Transmit path 3200 includes a first microphone 3201 and a secondmicrophone 3202. Each of first microphone 3201 and second microphone3202 may be an omni-directional microphone, a uni-directionalmicrophone, an adaptive microphone, or the like. First microphone 3201may be positioned on a telephone to pick up mostly a near-end user'svoice, similar to first microphone 201 described above. Secondmicrophone 3202 may be positioned on the telephone to pick up mostlybackground noise, similar to second microphone 202 described above.

In addition, transmit path 3200 includes an A/D converter 3210, an A/Dconverter 3212, a VAD 3230, a signal processor 3220, a speech encoder3204, a channel encoder 3205, a modulator 3206, an RF module 3207, andan antenna 3208. Speech encoder 3204, channel encoder 3205, modulator3206, RF module 3207, and antenna 3208 are respectively similar tospeech encoder 104, channel encoder 105, modulator 106, RF module 107,and antenna 108 discussed above with reference to transmit path 100 ofFIG. 1A, and thus their operation will not be discussed in detail below.

Importantly, VAD 3230 receives input from both first microphone 3201 andsecond microphone 3202, and therefore provides improved voice activitydetection as described above, for example, with respect to FIGS. 14 and15. VAD 3230 provides a voice activity signal to signal processor 3220and signal processor 3223. The voice activity signal indicates timeintervals in which the output of first microphone 3201 includes a voicecomponent (i.e., time intervals during which a near-end user talks).

Receive path 3240 includes a receiver module 3260, a decoder 3270, asignal processor 3223, a D/A converter 3222, and a speaker 3229.Receiver module 3260 is configured to receive signals from a far-enduser's telephone. Receiver module 3260 is similar to receiver module2660 of FIG. 26. In an embodiment, receiver module 3260 includes anantenna, an RF module, and a demodulator similar to antenna 128, RFmodule 127, and demodulator 126, respectively, which, as described abovewith reference to FIG. 1B, collectively function to receive transmittedsignals.

Decoder 3270 is coupled to receiver module 3260, and is configured todecode the signals received by receiver module 3260, thereby producingan output signal. For example, decoder 3270 can include a channeldecoder and speech decoder similar to channel decoder 125 and speechdecoder 124, respectively, which, as described above with reference toFIG. 1B, collectively function to decode a received signal.

Signal processor 3223 is coupled to decoder 3270, A/D converter 3212,and VAD 3230. Signal processor 3223 is configured to (i) analyze afar-end speech signal (received from receiver module 3260) with respectto a background noise signal (received from A/D converter 3212), and(ii) modify the far-end speech signal based on the analysis. Because VAD3230 provides improved voice activity detection, signal processor 3223can be configured to modify the far-end speech signal only during timeintervals in which the near-end user is not talking. Thus, signalprocessor 3223 provides improved speech intelligibility, while reducingthe “shouting match” problem described above.

In an embodiment, signal processor 3223 of receive path 3240 may beembodied in a single integrated circuit (e.g., chip) with signalprocessor 3220 of transmit path 3200. In another embodiment, signalprocessor 3223 of receive path 3240 and signal processor 3220 oftransmit path 3200 are embodied in separate integrated circuits (e.g.,chips).

D/A converter 3222 receives the output from signal processor 3223 toprovide an analog output signal.

Speaker 3229 receives the analog output signal from D/A converter 3222,and produces a pressure sound wave corresponding thereto. For example,speaker 3229 can be similar to speaker 129 (FIG. 1B) and/or speaker2680. Additionally, a power amplifier (not shown) can be included beforespeaker 3229 (or speaker 129) to amplify the output signal before it issent to speaker 3229 (speaker 129) as would be apparent to a personskilled in the relevant art(s).

C. Example Method for Providing Improved Speech Intelligibility inAccordance with an Embodiment of the Present Invention

FIG. 33 illustrates a flowchart 3300 of a method for providing improvedspeech intelligibility in a telephone having a first and a secondmicrophone in accordance with an embodiment of the present invention.For example, this method may be implemented to improve the speechintelligibility of a near-end user and/or a far-end user when thenear-end user is in a noisy environment.

The method of flowchart 3200 begins at a step 3301 in which a firstmicrophone (such as first microphone 3201 of FIG. 32) outputs a firstaudio signal. The first audio signal includes (i) a voice component of anear-end user when the near-end user talks, and (ii) a background noisecomponent.

In a step 3302, a second microphone (such as second microphone 3202 ofFIG. 32) outputs a second audio signal. The second audio signal maycomprise mostly a background noise component.

In a step 3303, a ratio between the first audio signal and the secondaudio signal is monitored. The ratio may comprise, for example, anenergy ratio, an average magnitude ratio, or the like. In an embodiment,VAD 3230 monitors the ratio to detect voice activity (e.g., timeintervals in which the near-end user speaks) as described above, forexample, with respect to FIGS. 14 and 15.

In a step 3304, a voice activity signal is generated responsive to theratio. For example, VAD 3230 may generate the voice activity signal. Inthis example, the voice activity signal is provided to signal processor3220 of transmit path 3200 and signal processor 3223 of receive path3240, as illustrated in FIG. 32. The voice activity signal providesimproved detection of voice activity compared to conventional VADsbecause this voice activity signal is generated responsive to a ratiobetween the first and second audio signals, as described above, forexample, with respect to FIGS. 14 and 15.

In a step 3305, a third audio signal is received. The third audio signalcomprises a voice component of a far-end user, and may be received, forexample, by receiver module 3260 of receive path 3240.

In a step 3306, the third audio signal is modified responsive to thevoice activity signal. For example, signal processor 3223 may boost theentire third audio signal, or specific frequency ranges of the thirdaudio signal, as described above. By using the outputs of both the firstand second microphones (e.g., microphones 3201 and 3202), the voiceactivity signal provides a more reliable indication of the voiceactivity included in the first audio signal output by the firstmicrophone. The more reliable voice activity signal reduces the chanceof the “shouting match” problem described above because the third audiosignal can be boosted only during time intervals in which the near-enduser is not talking, for example.

In an embodiment (not illustrated in FIG. 33), active noise cancellationis used to reduce the noise in the output of the first microphone (e.g.,first microphone 3201). The active noise cancellation may be implementedin a similar manner to the active noise cancellation described above,for example, with respect to FIGS. 5-9. As a result, the signal-to-noiseratio (SNR) of the first audio signal will be increased, thereby makingthe near-end user's voice easier to understand by the far-end user.

In another embodiment, at least one of the first and second microphonescomprises an adaptive microphone array. For example, the firstmicrophone may comprise a first microphone array (similar to firstmicrophone array 201′ of FIG. 24). In this example, the first microphonearray can be steered toward the near-end user's mouth to get better SNRfor the near-end user's voice. That is, the first microphone array canbe steered so that the near-end user's mouth is within a maximumsensitivity angle of the first microphone array.

As a second example, the second microphone may comprise a secondmicrophone array (similar to second microphone array 202′ of FIG. 24).In this example, the second microphone array can be steered away fromthe near-end user's mouth to minimize the voice pick up in the noisesignal. That is, the second microphone array can be steered so that thenear-end user's mouth is within a minimum sensitivity angle of thesecond microphone array.

In a further embodiment, the microphones or microphone arrays of theforgoing embodiments may be used in conjunction with active noisecancellation at the near-end user's ears through the loudspeaker of thetelephone or the loudspeakers of the two-ear headset for the telephone.This improves the intelligibility of the far-end user's voice to thenear-end user.

To prevent the near-end telephone user from thinking that the telephoneis dead or the phone line is disconnected, most telephones provide aso-called “side-tone” by adding a scaled down version of the signalpicked up by the microphone to the signal that is to be played backthrough the loudspeaker so that the near-end user can hear his or herown voice while he or she talks. However, if the near-end user is in ahighly noisy environment, such a side-tone scheme can play back throughthe loudspeaker the ambient acoustic noise picked up by the microphone,thus making it more difficult for the near-end user to understand thefar-end user's voice.

In a further embodiment of the present invention (not specifically shownin FIG. 32), the signal processor 3223 takes the output signal of thefirst microphone 3201 through A/D 3210, reverses the signal polarity andscales the signal magnitude, adds the resulting signal to the normallyprocessed version of the output signal of the decoder 3270, and thensends the resulting sum signal to the D/A 3222. With thispolarity-reversed side-tone arrangement, when the near-end telephoneuser talks, he or she is still able to hear his or her own voice asside-tone. On the other hand, the polarity-reversed side-tone tends toat least partially cancel the ambient acoustic noise at the ear drum ofthe near-end user, thus making it easier for the near-end user tounderstand the far-end user's voice.

XII. CONCLUSION

The specifications and the drawings used in the foregoing descriptionwere meant for exemplary purposes only, and not limitation. It isintended that the full scope and spirit of the present invention bedetermined by the claims that follow.

1. A method in a telephone for improving speech intelligibility when thetelephone is used in an environment having background noise, the methodcomprising: outputting a first audio signal from a first microphone, thefirst audio signal comprising a voice component when a near-end usertalks and a background noise component; outputting a second audio signalfrom a second microphone; monitoring a ratio between the first audiosignal and the second audio signal; generating a voice activity signalresponsive to the ratio between the first audio signal and the secondaudio signal, the voice activity signal identifying time intervals inwhich the first audio signal includes the voice component of thenear-end user; receiving a third audio signal comprising a voicecomponent of a far-end user; and modifying the third audio signalresponsive to the voice activity signal.
 2. The method of claim 1,wherein the modifying comprises: boosting the third audio signalresponsive to the voice activity signal.
 3. The method of claim 1,wherein the modifying comprises: comparing a frequency spectrum of thethird audio signal to a frequency spectrum of the second audio signal;and boosting at least one frequency of the third audio signal responsiveto the voice activity signal.
 4. The method of claim 1, furthercomprising: canceling the background noise component of the first audiosignal based on the content of the first audio signal and the secondaudio signal.
 5. The method of claim 1, further comprising: modifyingthe third audio signal based on the content of the third audio signaland the content of the second audio signal; and generating a sound wavefrom a loudspeaker based on the modified third audio signal, the soundwave reducing the background noise of the environment proximate to theloudspeaker.
 6. The method of claim 1, wherein the first microphonecomprises a microphone array.
 7. The method of claim 6, whereinoutputting the first audio signal from the microphone array comprises:(a) outputting an audio signal from each microphone in the microphonearray; (b) detecting a direction of arrival (DOA) of a sound waveemanating from the mouth of the near-end user based on the audiosignals; and (c) adaptively combining the audio signals based on the DOAto produce the first audio signal.
 8. The method of claim 7, whereinstep (c) comprises: effectively steering a maximum sensitivity angle ofthe microphone array so that the mouth of the near-end user is withinthe maximum sensitivity angle, wherein the maximum sensitivity angle isdefined as an angle within which a sensitivity of the microphone arrayis above a predetermined threshold.
 9. The method of claim 1, whereinthe second microphone comprises a microphone array.
 10. The method ofclaim 9, wherein outputting the second audio signal from the microphonearray comprises: (a) outputting an audio signal from each microphone inthe microphone array; (b) detecting a direction of arrival (DOA) of asound wave emanating from the mouth of the near-end user based on theaudio signals; and (c) adaptively combining the audio signals based onthe DOA to produce the second audio signal.
 11. The method of claim 10,wherein step (c) comprises: effectively steering a minimum sensitivityangle of the microphone array so that the mouth of the near-end user iswithin the minimum sensitivity angle, wherein the minimum sensitivityangle is defined as an angle within which a sensitivity of themicrophone array is below a predetermined threshold.
 12. A telephone forimproving speech intelligibility when the telephone is used in anenvironment having background noise, the telephone comprising: a firstmicrophone that outputs a first audio signal, the first audio signalcomprising a voice component when a near-end user talks and a backgroundnoise component; a second microphone that outputs a second audio signal;a voice activity detector (VAD) that (i) monitors a ratio between thefirst audio signal and the second audio signal, and (ii) generates avoice activity signal responsive to the ratio between the first audiosignal and the second audio signal, wherein the voice activity signalidentifies time intervals in which the first audio signal includes thevoice component of the near-end user; a receiver module that receives athird audio signal, the third audio signal comprising a voice componentof a far-end user; and a signal processor that modifies the third audiosignal responsive to the voice activity signal.
 13. The telephone ofclaim 12, wherein the signal processor comprises: signal processinglogic that boosts the third audio signal responsive to the voiceactivity signal.
 14. The telephone of claim 12, wherein the signalprocessor comprises: signal processing logic that (i) compares afrequency spectrum of the third audio signal to a frequency spectrum ofthe second audio signal, and (ii) boosts at least one frequency of thethird audio signal responsive to the voice activity signal.
 15. Thetelephone of claim 12, further comprising: a noise cancellation modulethat cancels the background noise component of the first audio signalbased on the content of the first audio signal and the second audiosignal.
 16. The telephone of claim 12, further comprising: a noisecancellation module that modifies the third audio signal based on thecontent of the third audio signal and the content of the second audiosignal; and a loudspeaker that generates a sound wave based on themodified third audio signal, the sound wave reducing the backgroundnoise of the environment proximate to the loudspeaker.
 17. The telephoneof claim 12, wherein the first microphone comprises a microphone array.18. The telephone of claim 17, wherein each microphone of the microphonearray outputs an audio signal, and the telephone further comprises: adigital signal processor (DSP) that (i) detects a direction of arrival(DOA) of a sound wave emanating from the mouth of the near-end userbased on the audio signals output by the microphones of the microphonearray, and (ii) adaptively combines the audio signals based on the DOAto produce the first audio signal.
 19. The telephone of claim 18,wherein the DSP effectively steers a maximum sensitivity angle of themicrophone array so that the mouth of the near-end user is within themaximum sensitivity angle, wherein the maximum sensitivity angle isdefined as an angle within which a sensitivity of the microphone arrayis above a predetermined threshold.
 20. The telephone of claim 12,wherein the second microphone comprises a microphone array.
 21. Thetelephone of claim 20, wherein each microphone of the microphone arrayoutputs an audio signal, and the telephone further comprises: a digitalsignal processor (DSP) that (i) detects a direction of arrival (DOA) ofa sound wave emanating from the mouth of the near-end user based on theaudio signals output by the microphones of the microphone array, and(ii) adaptively combines the audio signals based on the DOA to producethe second audio signal.
 22. The telephone of claim 21, wherein the DSPeffectively steers a minimum sensitivity angle of the microphone arrayso that the mouth of the near-end user is within the minimum sensitivityangle, wherein the minimum sensitivity angle is defined as an anglewithin which a sensitivity of the microphone array is below apredetermined threshold.
 23. A method in a telephone for improvingspeech intelligibility when the telephone is used in an environmenthaving background noise, comprising: outputting a first audio signalfrom a first microphone, the first audio signal comprising a voicecomponent when a near-end user talks and a background noise component;reversing a polarity of the first audio signal to form a second audiosignal; receiving a third audio signal comprising a voice component of afar-end user; and forming a side-tone based on the second audio signaland the third audio signal.
 24. A telephone for improving speechintelligibility when the telephone is used in an environment havingbackground noise, comprising: a first microphone that outputs a firstaudio signal, the first audio signal comprising a voice component when anear-end user talks and a background noise component; a signal processorthat reverses a polarity of the first audio signal to form a secondaudio signal; a receiver module that receives a third audio signalcomprising a voice component of a far-end user; and a speaker thatoutputs a side-tone that is formed based on the second audio signal andthe third audio signal.